The present invention relates to a method of making an optical fiber array used for connecting optical fibers to an optical device such as an optical waveguide, and particularly to a method of making an optical fiber array in which bare fibers are fixed on a V-groove substrate.
In techniques for connecting optical fibers to an optical device such as an optical waveguide, there is known a method in which connection is made by using an optical fiber array. FIGS. 12A and 12B are views for explaining an example in which an optical fiber array is comprised of optical fibers of a fiber ribbon, and the optical fibers are connected to optical waveguides. FIG. 12A is a side view and FIG. 12B is a plan view. The optical waveguides and optical fibers inside a waveguide chip and the optical fiber array are shown by thin lines. In the drawings, reference numerals 11 and 11' designate optical fiber arrays, 12 and 12' designate eight-core fiber ribbons, 13 designates an optical waveguide chip, and 14 designates an optical waveguide. Both end faces of the optical waveguide chip 13 are polished so that they are inclined by 8.degree. with respect to a plane vertical to an optical axis of the optical waveguide, and the optical waveguide chip 13 and the optical fiber arrays 11 and 11' are aligned and are bonded to each other. In the optical waveguide chip 13, four pairs of couplers are formed by the optical waveguides 14, and eight ports of the couplers at both sides are coupled to the fiber ribbons 12 and 12' of the optical fiber arrays 11 and 11' at both sides, respectively.
Increase in density, namely, increase in the number of fibers and decrease in volume,has been required for such an optical fiber array. A perspective view of FIG. 13 shows an example of an optical fiber array disclosed in a paper C-335 by Oguchi, et al, p. 4-332, Spring meeting of the Institute of Electronics, Information and Communication engineers, 1994, paper C-3-15 by Takagi, et al, p. 200, General meeting of the Institute of electronics, Information and communication engineers, 1997, Japanese Patent Unexamined Publication No. Hei. 9-68629, and Japanese Patent Unexamined Publication No. Hei. 9-230158. In the drawing, reference numerals 15 and 16 designate fiber ribbons, 17 designates a V-groove substrate, and 18 designates a fiber pressing member. The coating of tip portions of the two fiber ribbons 15 and 16 are removed so that the optical fibers of the fiber ribbons are made to be bare fibers, the fiber ribbons overlap each other while being shifted by half of an arrangement pitch of the optical fibers in the fiber ribbon, the bare fibers are disposed on the V-groove substrate 17 so that they are alternately arranged, the bare fibers are held by the fiber pressing member 18, and they are fixed by an adhesive. After adhesion, the end face is obliquely polished so that it makes an angle of, for example, 8.degree. with respect to a plane vertical to an optical axis of an optical waveguide.
FIG. 14 shows a state where two optical fiber arrays of FIG. 13 are arranged in parallel, and two pairs of two fiber ribbons overlapped up and down with respect to the V-groove substrate 17 are used. In the above paper C-3-15 by Takagi, et al, p. 200, General meeting of the Institute of electronics, Information and communication engineers, 1997, an optical fiber array of this structure is illustrated.
In the manufacture of such an optical fiber array, a V-groove substrate is used so as to accurately position and arrange bare fibers. If the V-groove substrate is not used but the bare fibers are arranged on a flat plate, fluctuations in the diameter of the bare fibers accumulate, so that a disadvantage is apt to occur especially when a plurality of bare fibers are arranged.
If the V-groove substrate is used and bare fibers are set in predetermined grooves, although an optical fiber array with an accurate pitch is obtained, the operation of arranging the bare fibers and setting them in the predetermined V grooves is difficult. The operation is more difficult in a case of a high density optical fiber array in which two fiber ribbons overlap each other. This point will be explained with reference to FIG. 15.
The coating of the tip portions of the two fiber ribbons 4 and 5 are removed so that bare fibers are exposed, and they are overlapped up and down with respect to V-groove substrate 1 while being shifted by a half pitch of an arrangement pitch of optical fibers in the fiber ribbon as shown in the uppermost figure of FIG. 15. The respective bare fibers of the two fiber ribbons are alternately arranged as shown in the middle figure of FIG. 15.
However, the bare fibers of the tip portions of the two fiber ribbons 4 and 5 are not necessarily arranged at an equal interval because of a slight deviation of a tape fiber and a shift in arrangement after removal of the common coating. In the drawing, they are shown irregularly with exaggeration. Because of such irregularity, electrostatic force, and the like, the alternately arranged bare fibers are not necessarily arranged in a close contact state. Rather, as shown by bare fibers 6', it is common that they are arranged in an irregular state. Thus, when the bare fibers 6' are arranged on the V-groove substrate 1, as shown by bare fibers 6, there appears a bare fiber which does not fall in the V groove.
Besides, after the bare fibers of one of the fiber ribbons are set in every other V groove, the bare fibers of the other fiber ribbon cannot be appropriately set in the remaining vacant grooves therebetween, since the bare fibers of the tip portions of the two fiber ribbons 4 and 5 are not necessarily separated at an even interval.
Further, since the outer diameter of a bare fiber is normally made 125 .mu.m, when the pitch of V grooves is made 125 .mu.m, the V grooves become shallower than those with 250 .mu.m pitch of a normal optical fiber array. Thus, it becomes more difficult to set the bare fibers in predetermined V grooves.
Thus, if there are attempts to automate this operation, a handling mechanism of an optical fiber for that requires accuracy of several .mu.m or less, and becomes expensive.
Further, since a bare fiber and a bare fiber are very close to each other, it is also difficult to recognize the set state of the bare fibers, and even if the set state is recognized, it is also difficult to correct that to correct that manually and it is more difficult to automate the correction thereof
Incidentally, the term "up and down" does not mean up and down in a positional relation with respect to the direction of the force of gravity, but up and down in a positional relation seen from the V-groove substrate. Thus, in the case where the V-groove substrate is placed horizontally, two fiber ribbons are placed in an up and down positional relation with respect to the direction of the force of gravity. The term "up and down" in the claims is also used with the same meaning.
The reason why an assembling technique of an optical fiber array has not been completed is that as described above, it is difficult to arrange bare fibers in V grooves.
Incidentally, although Japanese Patent Unexamined Publication No. Sho. 63-163406 discloses an example in which bare fibers are arrayed by using an optical fiber arraying member, this technique is such that the sides of bare fibers arranged on a flat plate are pushed so that the bare fibers are laterally shifted on the plane to be arrayed. In the case where the V-groove substrate is used, after a bare fiber is once set in some V groove, it is difficult to shift the fiber laterally. Thus, it is impossible to use a method disclosed in Japanese Patent Unexamined Publication No. Sho. 63-163406 for the purpose of arranging the bare fibers by using the V-groove substrate.